If you want to tag boundaries representing a closed ring/area please see Relation:boundary

Boundary segments are made with the generic relations: type=multilinestring and are used for grouping ways to form a bigger logical linear boundary making the border between two regions.
Such as the border between Italy and France but can also be used on any type of boundaries (administrative, protected_area, military, maritime, etc.)

A lot of people at this moment rely on the current approved relation system for mapmaking purposes

It requires an enormous change management project to make this work for all country/region/city etcetera borders

It doesn't solve the real problem: changing borders should require a verification process by an editor because border relations are broken up too often; OSM is yet (2012) far away from this Wikipedia level of matureness